1. Field of the Invention
The invention relates to drop-out compensation circuit arrangements for sound-reproducing apparatus in which a sound signal carried on a recording medium is played back, the medium being scanned by a pickup device and subsequently reproduced acoustically.
2. The Prior Art
U.S. Pat. No. 3,356,794 to Felix concernes reproducing circuits for wide-band magnetic recorders utilizing frequency modulation of a carrier with a video or other wide-band signal. As described in detail in the 3,356,794 patent, objectionable bursts of noise are produced by the frequency demodulator when the signal level from the magnetic medium falls off to a low, substantially non-existent level such, as, for example, during signal dropouts. In order to avoid such objectionable noise bursts the variable gain amplifier-limiter associated with the frequency demodulator circuit receives the signal from the transducer as well as a supplementary signal from an oscillator. The oscillator provides a constant signal which is at approximately one tenth the amplitude of the normal video signal from the transducer. The amplifier-limiter has a relatively low gain when the normal video signal is present, but changes to a relatively higher gain when only the oscillator signal is present. The oscillator signal prevents system noise from being demodulated and causes the output of the frequency demodulator to be maintained at an acceptable level such that drop-outs of the signal will not result in noise bursts which are noticeable as visible streaks and flashes to the viewer. It is to be noted that the Felix disclosure does not concern narrow-band signals, such as audio signals, nor does it attempt to remove completely the background noise from oscillator 20 during those periods when no drop-out is present.
U.S. Pat. No. 3,407,266 to Araki et al is concerned with the video drop-out which occurs in a helical-scan recorder when track-switching takes place. The Araki et al device produces two signal transducing heads which are spaced apart by an integral number of horizontal line lengths. When track-switching takes place, the output portion of the reproduction electronics is switched from one head to the other so that a few adjacent lines of the video signal are inserted into the track-switching drop-out interval. In this manner the synchronization pulses are maintained over the drop-out interval. A disc having an opening is rotated along with the heads so that a photocell will sense light passing through the opening and provide a switching signal at a predetermined time relative to track-switching. The signal from the photocell is used to momentarily switch the playback electronics coming from one head to the other. Those skilled in the art will recognize from the discussion below that the track-switching signal from the photocell of the Araki et al patent may be advantageously used in the circuitry of the present invention, as well. As will be described in greater detail below, the present invention is concerned with providing a suitable audio signal level during track-switching drop-outs of the type concerned in the Araki et al disclosure.
U.S. Pat. No. 3,699,246 to Hodge teaches means for detecting drop-outs in video signals and for switching in a delayed version of the signal during the drop-outs. Such technique is well known in the art and performs at least two useful functions. First, synchronization pulses are maintained during the drop-out interval and, secondly, streaking and flashes in the video signal are eliminated by substituting an acceptable signal level for objectionable noise levels. The delayed video lines which are substituted for objectionable noise during the drop-out intervals are not particularly noticeable to the viewer since there is considerable similarity between adjacent video lines.
In addition to the teachings of the foregoing patents which are concerned only with drop-outs in wide-band, or video, signals, several prior art patents are known which concern problems of noise bursts or drop-outs in narrow-band, or audio, signals. For example, German Patentschrift No. 865,068 relates to the problem of bridging brief sound gaps which occur in the audio signal reproducing channel of a movie film projector at splice points in the film. Such sound gaps are brief, but noticeable to the listener, if no steps are taken to mask or bridge the gap. German Patentschrift No. 865,068 points this out and provides for insertion in the sound gap of a noise signal which is approximately the same in level as the audio signals which precede and succeed the gap or splice points. This is accomplished by adding to the film sound track a segment at the desired noise level.
The present inventors have found that drop-outs in the signal from the recording medium transducer in a frequency modulated audio signal channel results in objectionable audio-frequency noise bursts at the output of the frequency demodulator. While such noise bursts may be blanked out entirely as taught, for example, in U.S. Pat. No. 2,288,000 to Kelly, the present inventors have found that total blanking out of such noise bursts results in sound gaps similar to thosd discussed in German Patentschrift No. 865,068, and such sound gaps are noticeable to the listener. Brief signal gaps or objectionable noise bursts occur with such regularity in recording in recording systems taught, for example. In U.S. Pat. Nos. 3,958,272 and 3,407,266, among others, that steps must be taken to render these occurrences innocuous to the listener if the recording systems are to be commercially acceptable.